This might be a dumb suggestion but you haven't actually mentioned that you've gone into settings > mobile data and turned on "Enable 4G".
Have you done that?
Done everyhing I could possibly think of. Finally got through to tech support from O2 and now they are blaming Apple.
Just tried swapping my sim with a friend's who is on Tesco and has 4G signal here:
My sim in her iPhone 5c - no 4G Her sim in my iPhone - no 4G
I don't understand...
@bq started
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This might be a dumb suggestion but you haven't actually mentioned that you've gone into settings > mobile data and turned on "Enable 4G".
Have you done that?